Many systems have been proposed in the past for providing manual steering of a vehicle having power steering in the event that the motor driven power steering pump should fail. To this end, it has been conventional to provide the hydrostatic steering unit with a hand pump. In the event that the motor driven pump should fail, the hand pump can be utilized to introduce fluid into the power steering cylinder or cylinders. A large heavy tractor must have a high steering ratio when steering manually in order to keep the steering wheel effort low enough to satisfy the manual mode requirement if the hand pump is connected with the steering cylinders in the same manner as when the motor driven pump is in operation. In this specification, the term "steering ratio" refers to the number of turns of the steering wheel required to accomplish a certain amount of steering, the steering ratio being higher when more turns of the steering wheel are required. This high ratio is satisfactory while operating manually, but a fewer number of steering wheel turns (i.e. a lower steering ratio) is desirable while operating in the power mode.
In U.S. Pat. No. 3,765,181 issued 16 Oct., 1973 a variable ratio steering system is disclosed which will operate one steering cylinder in the power steering mode if the steering resistance is low. However, if high steering resistance is encountered both steering cylinders will be operated at a steering ratio twice as high as the single cylinder operation. In the event that the motor driven pump should fail both steering cylinders may be operated manually at the same ratio as when both cylinders are operated in the power steering mode.
In the past, a dual steering ratio has been proposed. Accordingly, a first steering ratio would be utilized all the time when operating in the power steering mode, and, a second higher steering ratio would be employed when operating in the manual steering mode. Thus, it has been suggested to use two hydrostatic steering units in parallel with a slip clutch between. In such a system both hydrostatic steering motors would be used when operating in the power steering mode and only one hydrostatic motor would be employed when operating in the manual mode. Obviously, such a system would be more costly than a system employing only a single hydrostatic steering motor and would also be somewhat inefficient due to the work lost in the slip clutch.